Fluorescing compounds are a class of dyes that are rapidly finding their way into ink formulation and many other commercial applications. One feature of these compounds in ink systems (or other systems) is their ability to convey information by fluorescing at the region of the electromagnetic spectrum between 200 and 1100 nanometers. The ink with a low concentration of dye, when printed, is invisible to normal vision. It is used to mark paper (and other media) with indicia containing information. Upon exposure to the radiation of a specific wavelength, the dye component of the ink fluoresces at some specific, higher wavelength. The emitting radiation can be interpreted by appropriate detection where the resulting signals reveal coded information.
FIG. 1 is a diagrammatic, not to scale view showing a fluorescing ink forming an indicia 12 on a sheet of paper or other print medium 10. An illumination light source 14 directs the excitation radiation 16 onto the ink indicia 12. The ink emits fluorescent radiation 18 in response to the excitation, which is detected by the detector 20.
Exemplary inks are described in co-pending application entitled LIGHT SENSITIVE INVISIBLE INK COMPOSITIONS AND METHODS FOR USING THE SAME, application Ser. No. 09/181,581, filed Oct. 28, 1998, the entire contents of which are incorporated herein by this reference. Exemplary systems for reading fluorescing ink indicia are described in co-pending application Ser. No. 09/181,589, filed Oct. 28, 1998, entitled INTEGRATED PRINTING/SCANNING SYSTEM USING INVISIBLE INK FOR DOCUMENT TRACKING.
It can be desirable to mark both sides of a sheet media with indicia. For example, marking both sides of a single sheet of media is useful on special ink jet media to determine media type, side to print on, media size, media orientation in the printer and other information. Such a print media indicia marking technique is described in above-referenced co-pending application TAPE INDICIA ON CLEAR FILM MEDIA. This application describes machine readable indicia formed on a tape applied to the leading edge or other locations of a clear or transparent print medium. In one example, indicia are placed on both sides of the tape.
When both sides of a sheet of media or an indicia-bearing tape as described in the above-referenced co-pending application are printed with fluorescing inks, reading from one side of the sheet can interfere with reading from the other side of the sheet. This occurs when the fluorescent radiation leaks through the sheet or tape from ink printed on the back of the sheet or tape. This is illustrated in FIG. 2, which shows respective indicia 12A and 12B applied on opposite sides of the sheet 10, emitting respective fluorescent radiation 18A and 18B which is detected by the detector 20. Since different information can be coded on each side of the sheet 10, the detector will receive scrambled information and thus there will be confusion during the decoding process.
The leakage of emitting fluorescing radiation through a print medium or indicia-bearing tape changes the signal to noise ratio, which in turn requires more ink or ink containing more dye. This increases the visibility of marks and the cost of ink. Using a more sensitive detector also increases the cost of the detection system.
It would be advantageous to prevent this kind of leakage. A further advantage would be to provide a coding system in which an indicia-bearing tape applied to a transparent medium such as an overhead transparency has indicia readable from each side without interference from the indicia on the other side.
It would also be an advantage to provide a means of placing indicia which can be read from a simple, inexpensive detector.